This invention is related to a method and apparatus for dispensing photoresist onto a semiconductor wafer. More particularly, this invention relates to a method and apparatus for minimizing the amount of photoresist needed to form a photoresist layer of uniform thickness.
Conventional approaches for forming a layer of photoresist on a wafer involve flooding the center of the wafer with photoresist, then spinning the wafer. Spinning causes the photoresist to spread radially over the wafer, with excess photoresist being spun off. However, spinning typically causes the photoresist to be thicker at the wafer edges.
During spinning, surface tension forces hold photoresist to the wafer edge. The result is a buildup of photoresist around the edge of the wafer. To reduce such buildup a solvent often is squirted onto the bottom of the wafer during the spinning. The spinning forces the solvent to an underlying edge where photoresist is contacted, pulling some of the photoresist over the edge. The solvent, however, has not been found to leave a uniform coating of photoresist. Accordingly, a method for providing a more uniform thickness throughout the wafer surface is needed.
Another problem with the conventional spinning method is that approximately 99.5% of the photoresist applied to the wafer is spun off the wafer as waste. The excessive amounts of photoresist have been necessary, however, to prevent substantially non-uniform coatings. Reducing the amount of photoresist has been found to cause streaking or a "cobweb" effect. To avoid streaking and "cobwebs" conventional spinning methods use approximately 200 times the amount of photoresist required for the final photoresist layer. Accordingly, a more efficient method and apparatus for forming a uniform photoresist layer is needed.